A power amplifier (PA) circuit may be biased for different modes, or “classes” of operation. Exemplary classes include Class A, Class AB, and Class B. In Class A operation, a PA may be biased such that the PA is in a conducting, or ON, state during 100% of the cycle, or the entire cycle, of the input signal. The bias level is also typically selected such that the PA operates in the most linear portion of the transfer curve, which characterizes the PA circuit. In Class A operation, the output signal from the PA is typically a scaled version of the input signal, where the scaling factor is a function of the gain associated with the PA circuit. However, because of the bias level utilized for Class A operation, the PA is typically in a conducting state even when there is no input signal. Furthermore, even when the PA is amplifying an input signal, the efficiency of the PA may not exceed 50%. For example, each watt of delivered output power, or Pout, may require two (2) watts of delivered power, PDC, from a DC power supply source (such as a battery). One limitation of conventional Class A PA circuits for use in mobile wireless communication systems like wireless local area network (WLAN) systems is that high bias levels often utilized to enable large variations in output power levels may result in unacceptably short battery life and/or high levels of generated thermal heat.
In Class B operation, a PA may be biased such that the PA is in a conducting state during 50%, or half, of the cycle of the input signal. This may result in large amounts of distortion of the input signal in the output signal. In this regard, in Class B operation, the PA may operate in a nonlinear portion of the transfer curve. However, the theoretical efficiency of a Class B PA circuit may reach 78.5%. The higher efficiency of the Class B PA results from the PA being in a non-conducting, or OFF, state half of the time. While the PA is in the OFF state, power dissipation may be theoretically zero (0). One limitation of Class B PA circuits is that distortion levels in output signals may be unacceptably high.
In Class AB operation, a PA may be biased such that the PA is in a conducting state for greater than 50%, but less than 100%, of the cycle of the input signal. In Class AB operation, the PA may be more efficient than in Class A operation, but less efficient than in Class B operation. Furthermore, in Class AB operation, the PA may produce more distortion than in Class A operation, but less than in Class B operation.
In Class C operation, a PA may be biased such that the PA is in a conducting state for less than 50% of the cycle of the input signal. While Class C amplifiers may produce more distortion than Class A, Class AB, or Class B amplifiers, the theoretical efficiency of a Class C amplifier may reach 90%. The Class C amplifier may receive an input signal and generate a series of current pulse signals. The current pulse signals generated by the Class C amplifier may comprise undesired frequency components. The output signal from the Class C amplifier may be input to a tuned circuit, which may comprise circuitry to suppress unwanted frequency components. The resulting output signal from the tuned circuit may be a signal for which that comprises frequencies within a desired frequency band, for example such as a frequency band utilized in global system for mobile (GSM) communications systems.
While the operating class of a PA provides one measure of efficiency, another measure of efficiency is determined by how efficiently the output power from the PA, Pout, is delivered to a load. For purposes of the present application, this measure of efficiency may be referred to as load transfer efficiency. In a wireless communications system, an exemplary load may comprise an antenna. The PA may deliver the output power to the load most efficiently when the output impedance of the PA is equal to the impedance of the load. In this regard, the PA and the load may be referred to as being “impedance matched”.
Many conventional PA circuits are implemented in integrated circuit (IC) devices, or chips. The IC may comprise a die, which may comprise active and/or passive circuitry, and a package, which may comprise a plurality of pins, or contacts, which enable electrical conductivity between various contact points on the die, and various contact points on a board, or other electronic assembly on which the IC is installed.
Some conventional PA integrated circuit chips achieve impedance matching by insertion of an on-chip transformer between the output of the PA and a load, which is located off-chip. A transformer utilized for impedance matching may be referred to as a matching transformer. Because of limitations in on-chip transformer circuits, signal energy may be lost when coupling a signal from the primary windings of the on-chip transformer to the secondary windings of the on-chip transformer. The result may be a reduced level of delivered power to the load, Pload.
In a polar modulation system, the PA input may comprise a phase modulation signal and an amplitude signal. In response to the phase modulation signal, the PA may generate an output signal of constant amplitude for which the phase may vary at different time instants. In response to the amplitude modulation signal, the PA may vary the amplitude at different time instants for the signal generated in response to the phase modulation signal.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.